Coastal biogas to mitigate eutrophication and clean the coastline

The purpose of the action is to inform about best practises of anaerobic co-digestion of cast seaweed as means to mitigate eutrophication, clean the coastline, produce biogas and an organic fertiliser. We have found that collection of cast seaweed and anaerobic co-digestion in combination with the use of the digestate as an organic fertiliser is a cost-effective measure to mitigate eutrophication and reduce greenhouse gas emissions and at the same time provide a high number of other socio-economic benefits.

Solution to which problem

The Baltic Sea area suffers from eutrophication due to past and present excessive discharges of nutrients (nitrogen and phosphorus) to the marine environment. As a result, eutrophication causes a threat to the biodiversity of the Baltic Sea due to elevated levels of algal and plant growth, increased turbidity, oxygen depletion, changes in species composition and nuisance blooms of algae. Moreover, the increasing volumes of maritime biomass, which is permanently discarded on beaches from day to day or after thunderstorms, has caused serious local odour problem by cast seaweed fouling on the beaches. Consequently, extra financial and human resources are required for municipalities to cope with this issue. Additionally, unpleasant odours impact not only the recreational coastal areas but also the value of property built near the coastline.

The biggest problem is faced in Denmark and Sweden, followed by Germany. Therefore, Danish and Swedish local authorities together with communities have taken real action in the period from 2013 to 2015 trying to suppress the quantities of discarded seaweed on their shore. This was done by removing cast seaweed from beaches and utilising it for green energy production in two biogas plants. These attempts are helping to reduce eutrophication on the Baltic Sea area as well as to keep some recreational coastal areas clean and attractive for locals and tourists. While the demonstration in the Swedish pilot biogas plant suffered from sand accumulation in the digester and the operation has been stopped and the plant dismantled, the Danish biogas plant is not only in full operation its capacity has been increased.

Technical conditions

According to a local entrepreneur, the amount of cast seaweed collected from the 3.7 km coastline in Solrød Municipality in 2009 was equal to 4,000 tons which corresponded to 1,081 tons/year km. Moreover, the annual average collection of cast seaweed from the entire Køge Bay coastline (38.6 km) has been estimated to be 42,000 tons/year, which conforms to 1,141 tons/year km. Recently, the annual quantity of cast seaweed collected from 1-2 km of Køge Bay coastline corresponded to 1,500 tons/year. Moreover, around 1,200 tons of seaweed were collected in 2019.

The collected seaweed utilisation depends on the content of the heavy metal cadmium in the seaweed. Seaweed samples are taken once a month, and the results of the cadmium content become available in the middle of the month (it takes 7-8 working days to analyse the seaweed). If the cadmium content is below the limit value of 0.8 mg/kg dry matter, the seaweed is supplied to Solrød biogas plant. Otherwise, the seaweed is returned into the water. Correspondingly, works are carried out until the next measurement of the cadmium content in the seaweed. Typically, the cadmium limit value exceeds the threshold in the winter season.

During the feasibility study of Solrød biogas plant, the possibilities of seaweed collection were also studied. The prototype of a seaweed harvester was tested as a potential collection technique. It was found that this machine can collect approx. 30 m3 of cast seaweed per hour while the sand content remains 23–40% w/w of dry solids.


The collection of seaweed along the Køge Bay coastal line in Solrød municipality from 2016 to 2019 took place around the time of periods. Furthermore, the collection of cast seaweed can proceed continually during the whole year except for several cases, such as bad weather conditions and/or the concentration of cadmium is above the limit (0.8 mg/kg dry matter) in the collected seaweed. This prevents the collected seaweed to be transported to and used in the Solrød biogas plant for biogas production.

Seaweed undergoes pre-treatment one more time at the biogas plant. Firstly, seaweed is put into a receiving tank with a very strong stirrer, which separates seaweed from the sand residue. The sand is removed from the bottom of the tank. Further, purified seaweed is chopped finely in a macerator and mixed with liquid from the digester. Finally, the macerated seaweed together with the liquid is supplied via pipe systems into the biogas tank (digester) where gas formation takes place.

The development of the methods and tools, which are used to collect the seaweed, is still ongoing. Recently, seaweed has been collected and cleaned using a backhoe with a big shovel in front, a beach cleaning machine and a loader tractor. Also, a tractor with a giant rake and dump truck with iron bars are used to clear the ruts caused by collecting machines on the beach.


Benefits from the plant:

  • 60 GWh/year renewable energy production
  • 104 local jobs, of which 14 permanent
  • 40,100 tons CO2 saved per year (51% of the municipality target for 2025)
  • Sustainable waste treatment and lower costs of waste transport
  • Production of digestate as an organic fertiliser for farmers
  • Reduced leaching of N to aquatic environment by 62 tonnes/year (70% of requirement for Køge Bay)
  • Reduced leaching of P to the aquatic environment by 9 tonnes/year (100% of requirement for Køge Bay)
  • Reduced odour nuisance from the beach/seaweed
  • Improved sea water quality and higher recreational value of the maritime coastal area

This strategy allows collection around 90% of seaweed with less than 50% of sand. Moreover, the seaweed is now delivered continuously to the biogas plant instead of lying on the beach. It means that unpleasant odour is being reduced, spontaneous methane emissions from decaying seaweed are eliminated and the biogas production is improved.

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